Client/server computing has become more and more important over the past few years in the information technology world. This type of distributed computing allows one machine to delegate some of its work to another machine that might be, for example, better suited to perform that work. For example, the server could be a high-powered computer running a database program managing the storage of a vast amount of data, while the client is simply a desktop personal computer (PC) which requests information from the database to use in one of its local programs.
The benefits of client/server computing have been even further enhanced by the use of a well-known computer programming technology called object-oriented programming (OOP), which allows the client and server to be located on different (heterogeneous) "platforms". A platform is a combination of the specific hardware/software/operating system/communication protocol which a machine uses to do its work. OOP allows the client application program and server application program to operate on their own platforms without worrying how the client application's work requests will be communicated and accepted by the server application. Likewise, the server application does not have to worry about how the OOP system will receive, translate and send the server application's processing results back to the requesting client application.
Details of how OOP techniques have been integrated with heterogeneous client/server systems are explained in U.S. Pat. No. 5,440,744 and European Patent Published Application No. EP 0 677,943 A2. These latter two publications are hereby incorporated by reference. However, an example of the basic architecture will be given below for contextual understanding of the invention's environment.
As shown in FIG. 1, the client computer 10 (which could, for example, be a personal computer having the IBM OS/2 operating system installed thereon) has an application program 40 running on its operating system ("IBM" and "OS/2" are trademarks of the International Business Machines corporation). The application program 40 will periodically require work to be performed on the server computer 20 and/or data to be returned from the server 20 for subsequent use by the application program 40. The server computer 20 can be, for example, a high-powered mainframe computer running on IBM's MVS operating system ("MVS" is also a trademark of the IBM corp.). For the purposes of the present invention it is irrelevant whether the requests for communications services to be carried out by the server are instigated by user interaction with the first application program 40, or whether the application program 40 operates independently of user interaction and makes the requests automatically during the running of the program.
When the client computer 10 wishes to make a request for the server computer 20's services, the first application program 40 informs the first logic means 50 of the service required. It may for example do this by sending the first logic means the name of a remote procedure along with a list of input and output parameters. The first logic means 50 then handles the task of establishing the necessary communications with the second computer 20 with reference to definitions of the available communications services stored in the storage device 60. All the possible services are defined as a cohesive framework of object classes 70, these classes being derived from a single object class. Defining the services in this way gives rise to a great number of advantages in terms of performance and reusability.
To establish the necessary communication with the server 20, the first logic means 50 determines which object class in the framework needs to be used, and then creates an instance of that object at the server, a message being sent to that object so as to cause that object to invoke one of its methods. This gives rise to the establishment of the connection with the server computer 20 via the connection means 80, and the subsequent sending of a request to the second logic means 90.
The second logic means 90 then passes the request on to the second application program 100 (hereafter called the service application) running on the server computer 20 so that the service application 100 can perform the specific task required by that request, such as running a data retrieval procedure. Once this task has been completed the service application may need to send results back to the first computer 10. The server application 100 interacts with the second logic means 90 during the performance of the requested tasks and when results are to be sent back to the first computer 10. The second logic means 90 establishes instances of objects, and invokes appropriate methods of those objects, as and when required by the server application 100, the object instances being created from the cohesive framework of object classes stored in the storage device 110.
Using the above technique, the client application program 40 is not exposed to the communications architecture. Further the service application 100 is invoked through the standard mechanism for its environment; it does not know that it is being invoked remotely.
The Object Management Group (OMG) is an international consortium of organizations involved in various aspects of client/server computing on heterogeneous platforms with distributed objects as is shown in FIG. 1. The OMG has set forth published standards by which client computers (e.g. 10) communicate (in OOP form) with server machines (e.g. 20). As part of these standards, an Object Request Broker (called CORBA the Common Object Request Broker Architecture) has been defined, which provides the object-oriented bridge between the client and the server machines. The ORB decouples the client and server applications from the object oriented implementation details, performing at least part of the work of the first and second logic means 50 and 90 as well as the connection means 80.
As part of the CORBA software structure, the OMG has set forth standards related to "transactions" and these standards are known as the OTS or Object Transaction Service. Computer implemented transaction processing systems are used for critical business tasks in a number of industries. A transaction defines a single unit of work that must either be fully completed or fully purged without action. For example, in the case of a bank automated teller machine from which a customer seeks to withdraw money, the actions of issuing the money, reducing the balance of money on hand in the machine and reducing the customer's bank balance must all occur or none of them must occur. Failure of one of the subordinate actions would lead to inconsistency between the records and the actual occurrences.
Distributed transaction processing involves a transaction that affects resources at more than one physical or logical location. In the above example, a transaction affects resources managed at the local automated teller device as well as bank balances managed by a bank's main computer. Such transactions involve one particular client computer (e.g, 10) communicating with one particular server computer (e.g., 20) over a series of client requests which are processed by the server. The OMG's OTS is responsible for coordinating these distributed transactions.
Usually, an application running on a client process begins a transaction which may involve calling a plurality of different servers, each of which will initiate a server process to make changes to its local database according to the instructions contained in the transaction. The transaction finishes by either committing the transaction (and thus all servers finalize the changes to their local databases) or aborting the transaction (and thus all servers "rollback" or ignore the changes to their local databases). To communicate with the servers during the transaction (e.g., instructing them to either commit or abort their part in the transaction) one of the processes involved must maintain state data for the transaction. This usually involves the process to set up a series up objects, one of which is a coordinator object which coordinates the transaction with respect to the various servers.
Because there is always a chance of failure, the process must store the state data for the transaction in such a way that this state data is recoverable in the instance of a failure of the machine or process holding the state data. That is, in the event of system failure, this state data can be stored as a log file in external memory local to the machine or process holding the state data, just as the system is beginning to fail. After the problem giving rise to the failure has been identified and corrected, the system is revived and the state data is reloaded so that the transaction that was in progress can be completed.
When a client application begins the transaction, there are two possible architectural choices available for selecting the location of the transaction state data. The first is the so-called "recoverable client" where the state data is stored locally to the client process. In this case, the client must have enough function included within it so that it can act as a server, since it must be able to accept remote method invocations on objects created as part of the transaction state data. For example, each server involved in the transaction must register itself with the client.
The modern trend is to move away from adding function to clients and to have, instead, "pure" or "thin" clients which cannot act as servers. For example, this is the main thrust of the network computing paradigm which is steadily growing in popularity as clients are being given less and less function so that central changes can be made to server functionality without having to make the same changes to each client functionality. Also, the cost of client machines can be significantly reduced, as such machines do not require any storage at all (all storage is located at the remote servers). Thus, the second architectural choice for selecting the location of the transaction state data is the "non-recoverable client" where the transaction state data is stored off of (remote from) the client process (for example, on one of the servers).
Both of these architectural choices have their relative advantages and disadvantages. Therefore, it is highly advantageous to be able to support either architecture depending on the circumstances. However, when implementing a transaction the client application source code must be written in a different way depending on whether a recoverable client or a non-recoverable client is to be used. If the client is recoverable, the client application source code must include the code to create the transaction locally, and if the client is non-recoverable, the client application must include the code to create the transaction remotely.
Thus, a client application originally written for a recoverable client platform cannot easily be ported to a non-recoverable client platform. A source code client application can only be built and run in one architectural environment.
If the source code writer attempts to get around this problem, he/she faces a heavy burden because in order to write an architecture independent application, the client application would have to contain two versions of the application (one for each of the two architectures mentioned above), as well as code portions for distinguishing the architectures to decide which version to use in a given case. In providing this extra software code, the client programmer is distracted from his/her task of writing the substantive parts of the client application.